Field of the Invention
The present invention pertains generally to continuous wave modulation, and more particularly relates to a reduced complexity tracking algorithm for the symbol timing synchronization of a continuous phase modulation (CPM) input data signal using a Viterbi algorithm for demodulation.
Continuous phase modulation (CPM) is a modulation format which generates a constant envelope signal and which possesses advantageous spectral properties. CPM has found utility in systems where it is desirable to use a constant envelope modulation, such as where high power nonlinear amplifiers are used. The constant envelope constraint introduces memory at the transmitter, effectively making a CPM signal more difficult to detect. Typically a Viterbi algorithm is used to perform sequence detection on the received symbol sequence. In order to compute the proper branch metric signals required in the Viterbi algorithm, the receiver must have an accurate estimate of the phase of the transmitter's symbol clock signal with respect to the received signal. This is known as symbol timing synchronization. The present invention describes a reduced complexity-tracking algorithm for the symbol timing synchronization of a CPM modulated signal where the Viterbi algorithm is used for demodulation.
Trellis decoders or demodulators are frequently used to demodulate signals modulated by continuous phase modulation (CPM). In the context of the present invention, coding/decoding and modulation/demodulation are analogous, and may be viewed as corresponding. CPM modulation has an advantage of providing a signal having substantially constant power, which is a marked advantage when transmitting the modulated signals over a nonlinear channel, as the constant power tends to reduce the generation in such a channel of unwanted distortion products which obscure the signals. A further advantage of CPM modulation is that the bandwidth of the transmitted signal is easily maintained, and the frequency spectrum exhibits low sidelobes, which is advantageous for situations in which a plurality of signals traverse a channel, as the signal spectrum for one of the signals traversing the channel has little frequency overlap with the signal next adjacent in frequency. In other words, the channels may be closer together in frequency.
CPM modulation is performed, in general, by converting the information or signal to m-ary quantized form, if not already in the desired form. For the simple case in which m=2, the signal is converted into binary form. The m-ary signal is applied to a shift register array having a particular length. As the signal bits are applied to the input end of the shift register array, the previously-applied signals are clocked and propagate through the shift register array, altering the states of the registers in succession. A combinatorial or functional logic arrangement is coupled to the output of each register of the array, and applies some function to the combination of register logic states. The applied function results in one or more output signals, which depend upon the combinatorial function, and also depend upon the current state of each register in the array. The current state of each register in the array in turn depends upon the history of the input signal.
The demodulation of a signal modulated in the above-described fashion can be accomplished by a trellis demodulator. The “trellis” represents, by “nodes”, the possible states of the registers of the modulator, and by lines joining the nodes, the possible paths by which transitions between states can be made. The trellis demodulator is often implemented as a Viterbi algorithm which performs sequence detection on the received symbol sequence. Demodulation using a Viterbi algorithm requires an accurate estimate of the phase of the transmitter's symbol clock signal with respect to the received signal. The process of obtaining an accurate estimate of the phase of the transmitter's symbol clock signal with respect to the received signal is known as symbol timing synchronization.